(i) Field to which invention relates
The present invention is with respect to a column for heat and mass transfer between liquids and gaseous phases, that is to say vapors or gases, and made up of a column wall, spaced stage plates within said column wall placed one over the other, each stage plate having a ring part fixed to said column wall, and a wheel part supported in said column wall, and of a head inlet for liquid, the wheel parts being designed for centrifugally impelling liquid outwards forcing it over the ring part of each stage and then down onto the next stage plate thereunder on its way from the head inlet through a bottom of the column.
(ii) The prior art
The utility of a column is more specially dependent on the efficiency, the pressure drop, the possible working load and the range of possible change in characteristics of operation or adaptability. Efficiency is dependent on the degree of contact between the two phases (vapor and liquid) moving in counter-current and on the degree to which liquid is moved upwards by the vapor with it. The degree of contact goes up with the degree of turbulence between the phases, the turbulence in turn increasing the pressure drop.
The pressure drop between the column bottom and the column head may not be overgreat and has to be in line with conditions in industry. The upper limit to the operation temperature has to take into account the thermal stability of the product, this fixing however, the pressure in the bottom of the column. For each separation operation a certain number of stages (theoretical trays) is necessary, this controlling the pressure drop at each stage and for this reason the overall pressure drop. It will readily be seen from this that the efficiency and the pressure drop in the column might be made better by supplying energy to the column.
For this reason columns of the sort noted have been put forward (see Chemical Engineering 8.8.1960, pages 58, 82 and 84) in which the stages or trays are made of a fixed ring part and a wheel part in the middle. In the case of one first design of such a system the fixed ring parts are not placed one over the other--as is the case with a normal column with stage trays--but grouped together to take the form of a screw structure on which the liquid is transported downwards. In the open middle part of the screw structure or of the column a continuous shaft is placed on which spaced horizontal round plates are supported, between which and at some distance from the shaft generally axially running blades are placed. On turning the shaft the gaseous phase (gas or vapor) is aspirated in axially and forced generally radially outwards by the blades. At the same time the input of liquid takes place in such a way that it comes up against the blades and is impelled outwards. Because of the spraying of the liquid the pressure drop in the column is decreased while at the same time, because of the turning blades, there is a pressure increase in the gaseous phase. A shortcoming in this respect is however the complex design of the apparatus and the fact that it is hard for the thermodynamic or physical conditions to be kept under control from one stage to the next one so that, more specially, the separating effect is not in line with present day needs.
In the case of a further design on the lines noted, the fixed, unmoving ring part is made up of an overflow gutter fixed to the column and a ring-like gutter, fixed near to the middle of the apparatus and joined by way of arms with the overflow gutter. Within the ring-like gutter there is a wheel turned by a central shaft. In its parts over the ring-like gutter the wheel has openings and over these openings there are horizontal sheet metal guides on the outside of the wheel. On turning the wheel in the ring-like gutter the liquid within it is impelled centrifugally outwards, is forced upwards and comes out through openings in the gutter so that, because of the horizontal sheet metal guides a number of liquid veils, placed one over the other and generally horizontal, will be produced. The vapor or gas current is in an upward direction through the spaces between the arms and between the overflow gutter and the ring-like gutter, the gaseous phase then moving through the liquid veil. The liquid which is impelled outwards comes up against the wall of the column and then goes downwards in through the overflow gutter from which it goes onto the next stage plate. A part of the liquid, running onto the top side of the arm connections, goes back to the ring-like gutter. In the case of this column as well positive control of thermodynamic conditions is hard to take care of, because the liquid moving by way of the arm connections back into the ring-like gutter, that is to say which is only kept going in a small circuit without coming out of the circuit, is greatly dependent on the throughput and viscosity of the liquid. Furthermore it is not possible to make certain that the liquid undergoes mass transfer on every stage plate or tray, because an important part of the liquid, which comes from the overflow gutter, is not able to make its way downwards between the spaces between the arm connections of the next stage tray or plate. This amount of the liquid as well is greatly dependent on the throughput rate, the viscosity and the speed of the gaseous phase and furthermore on the conditions of flow.